The present invention relates generally to phased array antennas, and more particularly to microstrip patch antennas having coplanar waveguide (CPW) voltage-tuned phase shifters.
A phased array refers to an antenna having a large number of radiating elements that emit phased signals to form a radio beam. The radio signal can be electronically steered by the active manipulation of the relative phasing of the individual antenna elements. The electronic beam steering concept applies to antennas used with both a transmitter and a receiver. Electronically scanned phased array antennas are advantageous in comparison to their mechanical counterparts with respect to speed, accuracy, and reliability. The replacement of gimbals in mechanically scanned antennas with electronic phase shifters in electronically scanned antennas increases the survivability of antennas used in defense systems through more rapid and accurate target identification. Complex tracking exercises can also be maneuvered rapidly and accurately with a phased array antenna system.
Phase shifters play key role in operation of phased array antennas. Electrically controlled phase shifters can utilize tunable ferroelectric materials, whose permittivity (more commonly called dielectric constant) can be varied by varying the strength of an electric field to which the materials are subjected. Even though these materials work in their paraelectric phase above the Curie temperature, they are conveniently called xe2x80x9cferroelectricxe2x80x9d because they exhibit spontaneous polarization at temperatures below the Curie temperature. Tunable ferroelectric materials including barium-strontium titanate (BST) or BST composites have been the subject of several patents.
Dielectric materials including barium strontium titanate are disclosed in U.S. Pat. No. 5,312,790 to Sengupta, et al. entitled xe2x80x9cCeramic Ferroelectric Materialxe2x80x9d; U.S. Pat. No. 5,427,988 to Sengupta, et al. entitled xe2x80x9cCeramic Ferroelectric Composite Materialxe2x80x94BSTOxe2x80x94MgOxe2x80x9d; U.S. Pat. No. 5,486,491 to Sengupta, et al. entitled xe2x80x9cCeramic Ferroelectric Composite Materialxe2x80x94BSTOxe2x80x94ZrO2xe2x80x9d; U.S. Pat. No. 5,635,434 to Sengupta, et al. entitled xe2x80x9cCeramic Ferroelectric Composite Materialxe2x80x94BSTO-Magnesium Based Compoundxe2x80x9d; U.S. Pat. No. 5,830,591 to Sengupta, et al. entitled xe2x80x9cMultilayered Ferroelectric Composite Waveguidesxe2x80x9d; U.S. Pat. No. 5,846,893 to Sengupta, et al. entitled xe2x80x9cThin Film Ferroelectric Composites and Method of Makingxe2x80x9d; U.S. Pat. No. 5,766,697 to Sengupta, et al. entitled xe2x80x9cMethod of Making Thin Film Compositesxe2x80x9d; U.S. Pat. No. 5,693,429 to Sengupta, et al. entitled xe2x80x9cElectronically Graded Multilayer Ferroelectric Compositesxe2x80x9d; and U.S. Pat. No. 5,635,433 to Sengupta, entitled xe2x80x9cCeramic Ferroelectric Composite Materialxe2x80x94BSTOxe2x80x94ZnOxe2x80x9d. These patents are hereby incorporated by reference. A copending, commonly assigned United States patent application titled xe2x80x9cElectronically Tunable Ceramic Materials Including Tunable Dielectric And Metal Silicate Phasesxe2x80x9d, by Sengupta, filed Jun. 15, 2000, discloses additional tunable dielectric materials and is also incorporated by reference. The materials shown in these patents, especially BSTOxe2x80x94MgO composites, show low dielectric loss and high tunability. Tunability is defined as the fractional change in the dielectric constant with applied voltage.
Tunable phase shifters using ferroelectric materials are disclosed in U.S. Pat. Nos. 5,307,033, 5,032,805, and 5,561,407. These phase shifters include a ferroelectric substrate as the phase modulating elements. The permittivity of the ferroelectric substrate can be changed by varying the strength of an electric field applied to the substrate. Tuning of the permittivity of the substrate results in phase shifting when an RF signal passes through the phase shifter. The ferroelectric phase shifters disclosed in those patents suffer high conductor losses, high modes, DC bias, and impedance matching problems at K (18 to 27 GHz) and Ka (27 to 40 GHz) bands.
One known type of phase shifter is the microstrip line phase shifter. Examples of microstrip line phase shifters utilizing tunable dielectric materials are shown in U.S. Pat. Nos. 5,212,463; 5,451,567 and 5,479,139. These patents disclose microstrip lines loaded with a voltage tunable ferroelectric material to change the velocity of propagation of a guided electromagnetic wave. U.S. Pat. No. 5,561,407 discloses a microstrip voltage-tuned phase shifter made from bulk ceramic. Bulk microstrip phase shifters suffer from the need for higher bias voltage, complex fabrication processing and high cost.
Coplanar waveguides can also serve as phase shifters. U.S. Pat. Nos. 5,472,935 and 6,078,827 disclose coplanar waveguides in which conductors of high temperature superconducting material are mounted on a tunable dielectric material. The use of such devices requires cooling to a relatively low temperature. In addition, U.S. Pat. Nos. 5,472,935 and 6,078,827 teach the use of tunable films of SrTiO3, or (Ba, Sr)TiO3 with high a ratio of Sr. SrTiO3, and (Ba, Sr)TiO3 have high dielectric constants, which results in low characteristic impedance. This makes it necessary to transform the low impedance phase shifters to the commonly used 50m-ohm impedance.
U.S. Pat. No. 5,617,103 discloses a ferroelectric phase shifting antenna array that utilizes ferroelectric phase shifting components. The antennas disclosed in that patent utilize a structure in which a ferroelectric phase shifter is integrated on a single substrate with plural patch antennas. Additional examples of phased array antennas that employ electronic phase shifters can be found in U.S. Pat. Nos. 5,079,557; 5,218,358; 5,557,286; 5,589,845; 5,917,455; and 5,940,030.
It would be desirable to have a phased array antenna, which utilizes low cost phase shifters that can operate at room temperature and at high frequencies, such as above Ku band (12 to 18 GHz). This could play an important role in helping to make electronically scanned phased array antennas practical for commercial applications.
A phased array antenna includes a plurality of radiating elements, a feed line assembly, a ground plane positioned between the plurality of radiating elements and the feed line assembly, with the ground plane having a plurality of openings positioned between the plurality of radiating elements and the feed line assembly, and a plurality of voltage tunable dielectric phase shifters coupled to the feed line assembly.
Antennas constructed in accordance with this invention utilize low loss tunable film dielectric elements and can operate over a wide frequency range. The conductors forming the coplanar waveguide operate at room temperature. The devices herein are unique in design and exhibit low insertion loss even at frequencies in the above Ku band (12 to 18 GHz).